Experts flag growing underground stress

Three earthquakes hit near Dhaka in just 32 hours, raising concern as seismologists warn they could be foreshocks of a much stronger one.

The first, a strong 5.7-magnitude quake on Friday morning, killed 10 people in the capital, Narsingdi, and Narayanganj.

A second quake, measuring 3.3 on the Richter scale, was felt in Narsingdi's Palash around 10:36am yesterday. The third, a 3.7-magnitude tremor, followed at 6:06pm, according to a Met office bulletin.

 All three had their epicentres in Narsingdi, next to the capital.

Asked whether Bangladesh's many underlying fault lines raise the risk of more earthquakes, seismologist Prof Syed Humayun Akhter said the presence of fault lines alone is not the issue.

"The real concern is whether the energy accumulating in a subduction zone can cause a big quake," said Prof Humayun, who has studied earthquakes for 40 years.

Dhaka lies close to the Indo-Burma subduction zone, where the Indian plate dives beneath the Burma plate at the country's eastern margin. Experts have warned for nearly two decades that being situated near such a zone means sitting atop a seismic time bomb.

Seismologists say this vast zone, from Sylhet down to Teknaf, is steadily accumulating strain capable of triggering a major earthquake.

"The energy that was locked in this subduction zone [a place where one tectonic plate slides under another] for a long time is now being released in Narsingdi in small segments," said Prof Humayun.

He said it is impossible to forecast whether the next event will be a massive one.

"If the energy continues to come out in small segments like it did on Friday, then possibly there will be multiple small quakes. But we have calculated that the energy accumulated there could create a quake between magnitude 8.2 and 9," he added.

Another highly active zone is the Dauki fault, linked to several large earthquakes. Experts believe it ruptured three times in the last millennium, in 840, 920, and 1548.

The Great Assam Earthquake of 1897 shook the Indian subcontinent, including parts of Dhaka. More than a century later, scientists warn the region is overdue for another major event.

Dhaka remains deeply vulnerable because of its geology, density, and high economic exposure.

Smaller tremors are common. In the last decade, 583 earthquakes of magnitude 4 or above have struck within 300km of Bangladesh, an average of 58 a year, or four a month. On average, a quake occurs near Bangladesh every six days.

Experts say such smaller events can signal the build-up to a larger quake in regions with active faults such as the Dauki fault or the Indo-Burma subduction zone.

Prof Mehedi Ahmed Ansary of Buet's civil engineering department told The Daily Star that the return period for a magnitude 7 quake in the region is 125 to 175 years, and for a magnitude 8 quake, 250 to 300 years.

He said the timings of past major quakes, from the 8.5-magnitude Chittagong earthquake of 1762 to the 7.6-magnitude Sreemangal quake of 1918 and the 7.1-magnitude Dhubri quake of 1930, must be considered when assessing current risk.

"I think these earthquakes are foreshocks of a bigger one. We must be prepared for that," he said of the tremors felt on Friday and yesterday.

As one of the world's most densely populated cities, Dhaka is dangerously ill-prepared. If Friday's quake had been magnitude 7, Prof Ansary warned, casualties could have reached three lakh, and 35 percent of the city would have been destroyed.

"So we need to start checking our buildings according to the building code," he said.

He suggested classifying every building in Dhaka using a colour code -- green for structures built safely and according to code, yellow or orange for those at risk and in need of strengthening, and red for buildings that must be evacuated immediately.

It has been done in the US, Japan, and India, he said, adding that after the Rana Plaza disaster, Bangladesh implemented a similar process for the garments sector.